GB1564543A - Radiation curable coatings - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO RADIATION CURABLE COATINGS (71) We, DENNISON MANUFACTURING CO. LIMITED, a Company organised under the laws of England, of Colonial Way, Watford, Hertfordshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to radiation curable, and radiation cured, coatings for providing films, inks, varnishes, overcoatings and release coatings. It also relates to methods of making the coatings and suitable prepolymer materials for preparing them. The coatings are curable by various forms of "radiation", which is the term used for electromagnetic radiation, such as ultraviolet, and also for plasma arc and electron beam bombardment.

Coatings and inks for paper, foil, film, panels, tiles and other surfaces are desired which can be easily applied, are solvent and abrasion resistant, and are tough and flexible. It is also desirable to eliminate solvents, to reduce cost, avoid pollution, and achieve improved performance. In addition, if surfaces with low adhesion can be obtained, they can be used for transfer printing, temporary cover sheets to protect pressure-sensitive adhesives, and the expeditious handling of multiple articles in machinery or the like.

Existing radiation curable coatings, while useful for many purposes, do not sufficiently satisfy the foregoing criteria. Either they have limited strength, abrasion and solvent resistance, or they require long exposure times to radiation, especially ultraviolet light. Suitable low adhesion materials heretofore have required a solvent solution, or have had to be applied as hot melts.

It is an object of the invention to mitigate one or more of the foregoing disadvantages. It is believed that it is possible to produce, by means of the invention, release surfaces which can employ a wide variety of film-forming materials and which can be readily applied without substantial solvent and quickly cured by radiation, generally in one second or less under moderate exposure.

It is also believed possible to produce, by means of the invention, radiation curable materials, and coatings resulting therefrom, which cure rapidly under moderate exposure to give films with a desirable combination of toughness, flexibility and resistance to solvents and to abrasion. Such films are useful as inks, release coatings or protective varnishes.

According to the invention there is provided a radiation curable liquid prepolymer for preparing a radiation curable composition, the prepolymer comprising the reaction product of a polyamide polyamine and an unsaturated polybasic carboxylic acid or an anhydride or lower alkyl ester of such an acid.

In our co-pending British Patent Application No. 47458/78 (Serial No. 1564542) divided from British Patent Application No. 18911/76 (Serial No. 1564541) from which the present Application is also divided, there is described and claimed a valuable new class of release materials provided by including waxy materials in a radiation curable, film-forming liquid comprising a radiation curable liquid prepolymer in which the waxy material has limited compatability such that a thin layer thereof will migrate to the surface of a thin film of the liquid prior to cure. The waxy materials have good slip or release properties are generally lipophilic, and can comprise natural and synthetic waxes, oils, silanes, siloxanes particularly silicones and fluorocarbons. While generally non-reactive in the polymerizable liquid, they can contain reactive groups. For example, stearyl acrylate can be used with acrylate monomers and will migrate to the surface and orient with the stearyl groups towards the surface prior to reaction. The quantity of waxy material is not critical, provided enough is used to be effective, generally between about 0.5% and 10% by weight of the film-forming liquid.

Undue excess should be avoided to avoid property degradation in the film. About 3 % to 4% by weight is usually most preferred.

While waxes have been used with radiation curable materials to form an oxygen barrier and facilitate curing in air, they have not been used to provide release and transfer properties. The addition of a single wax component can provide high gloss coatings with release characteristics. The introduction of various mixtures of waxy materials has been found to give particularly suitable release properties.

The waxy materials above described can be incorporated in any suitable radiation curable liquid including prepolymers having ethylenic unsaturation, such as polyurethane/acrylate prepolymers, polyamides with a plurality of reactive amine groups reacted with ethylenically unsaturated polybasic carboxylic acids, acrylated epoxides, and epoxide resins catalytically polymerized with catalysts released under exposure to ionizing radiation.

Preferred radiation curable liquid prepolymers are known as modified polyamide prepolymers and are useful for overprint varnishes, for protective films on panels and tiles and for abrasion and solvent resistant inks.

Preferably, the starting material for the preparation of these modified prepolymers is a polyamide polyamine, which may be made by condensing an excess of polyamine with polycarboxylic acids, giving a still reactive condensation product. These are available in some variety, for example under the Registered Trade Marks Versamid and Emerez. The production of the polyamide polyamine may form part of the process for the production of the prepolymer. It is convenient, however, to use these polyamide polyamine materials as starting materials. The materials are sometimes known as reactive polyamide resins and contain primary and secondary amine groups.

The above-described starting material is preferably mixed with a smaller portion of an unsaturated dicarboxylic acid or an ethyl or methyl ester, or an anhydride of such an acid.

Preferably, the reaction mixture is stirred at a temperature such as 90"C for a few hours and a slow stream of nitrogen may be used to carry off the methanol or ethanol, which may be recovered by a cold trap. The reaction mixture is then conveniently stripped at low pressure (0.1 mm Hg), while stirring at 90"C to remove unreacted ester and more methanol or ethanol. Finally, the product is preferably allowed to cool in the presence of a small amount of p-methoxyphenol to inhibit premature polymerization. The ester that is recovered during low pressure stripping may be an isomerization product of the ester supplied to the reaction.

Thus, dimethyl maleate may be rearranged into dimethyl fumarate, and dimethyl itaconate may be partly isomerized to the mesaconate and the citraconate.

To prepare a radiation curable composition for use for example as a film, an ink, a varnish or a release coating the modified prepolymer as above prepared may then be compounded with one or more acrylate monomer esters on a roller mill. It is preferred to have trimethylolpropane triacrylate included as one of the components of this material, and of course sometimes this component is already there having been used as a solvent in the previous step.

Other acrylate monomer components suitable for use at this stage are acrylated epoxidized soybean oil, hydroxyethyl acrylate, hydroxyethyl methacrylate, 1,4-butanediol diacrylate, neopentyl glycol diacrylate, pentaerythritol tetra-acrylate, pentaerythritol triacrylate, hexanediol diacrylate, butyl acrylate, isodecyl acrylate, octadecylacrylate, dimethyl aminoethyl methacrylate, acrylic acid, methacrylic acid, acrylamide and/or methylene bis-acrylamide. It is preferred that the radiation curable liquid contain at least one acrylate monomer which is trifunctional (ethylenically unsaturated groups) or higher to promote cross-linking in the cured film. Acrylic monomers useful in this invention may include acrylic or methacrylic acids, preferably esters thereof, and preferably esters thereof condensed with polyols, and polyamines to form polyacrylates, that is monomers having two or more, and preferably three or more unsaturated acrylate groups.

It is also preferred to introduce at this stage a small to moderate amount of a photoinitiator for coatings to be cured by UV or plasma arc radiation (i.e. by actinic light generally).

Coatings to be cured by electron beam or X-ray radiation do not generally require a photoinitiator. The following are suitable photoinitiators: benzil, benzoin, benzoin alkyl ethers, acyloin derivatives in general, benzophenone, acetophenone and Michler's ketone.

Other compounds useful as photoinitiators for this purpose are those listed in Table 5-13, page 132, Molecular Photochemistry by N. J. Turro (W. Benhamin, Inc., 1967). Since acrylate monomers are essentially transparent to UV, sufficient photosensitizer should be used to permit rapid polymerization under moderate irradiation, preferably under 1 second.

From about 0.5 to about 20% by weight can be used with about 10% by weight of coating solution being preferred.

To provide a release coating or oxygen barrier there may be introduced at this stage generally between one half of 1% and a few percent of wax, e.g. between about 0.5% and about 10 % by weight, either paraffin wax, ester wax, a fluorocarbon wax or some other waxy material such as a higher alkyl alcohol or acid or oleamide, or waxy silicon-coating material such as a siloxane (including polysiloxanes) particularly silicone or silane or a mixture of some of these. Even a small quantity of such waxy material tends to migrate to the surface of the coating and provides a release type surface with characteristics similar to those of fluorocarbon polymers or silicones.

When the monomers added at this stage are such as to produce a mixture of low viscosity, the result is a coating material very useful as an overprint varnish. The coating can be cured by an exposure to ultraviolet radiation from a medium pressure mercury lamp for from 0.1 second to several seconds to form a hard glossy coating. When the monomers added are such as to produce a heavy oil, the resulting material is useful for coating rigid panels, such as paperboard or veneer panels, with a film having a thickness of one to several mils. In this case ultraviolet light might may be insufficient to provide quick curing and higher energy radiation can be used, such as electron beam radiation or X-rays. In the absence of air, an exposure to about 2-6 Mrad is sufficient for curing with electron beam radiation. In the presence of air, 10-20 Mrad or more are sometimes needed.

For the manufacture of inks, pigments such as Lithol rubine pigment, molybdate orange, chrome yellow, phthalocyanine blue, carbon black or dyes are mixed in at the same time as most of the acrylic monomers. Here also it is necessary to add 5-20% by weight of a photoinitiator to inks intended to be curable by UV or plasma arc radiation. The resulting ink may be printed onto paper, paperboard, plastics film metal or other stock, and the printing can be rapidly cured by exposure for a fraction of a second to ultraviolet light from a low pressure mercury lamp. An abrasion resistant and solvent resistant printing is thereby produced, which does not require an overprint varnish.

It has further been found advantageous to include in the radiation curable liquid solution an essentially linear polymer which is soluble therein, preferably one having a molecular weight of about 4,000 to the limit of solubility, and more preferably between about 10,000 and 20,000. Such addition has been found to improve the physical properties of the cured films by reinforcement and to limit shrinkage. This is especially useful with acrylate monomers which may have relatively high shrinkage on curing and which are highly cross-linked.

The foregoing aspects of the invention are illustrated in detail in the following examples wherein all parts are by weight. Examples 1 to 7 illustrate the preparation of various prepolymers in accordance with the invention and Examples 8 to 11 illustrate various coatings using the prepolymers of the previous Examples. Examples 12 to 30 illustrate particularly release-type coatings using prepolymers according to the invention.

EXAMPLE 1 205 parts of a reactive polyamide resin (derived from the condensation of polymerized fatty acids with polyalkylamines and containing primary and secondary amine groups; General Mills Chemicals, Versamid 115, amine number 238) and 30 parts of dimethyl male ate are mixed in a vessel equipped with an agitator. The vessel is swept with a slow stream of nitrogen while the reaction mixture is stirred at 90"C. for 3 hours. During this time 3 parts of methanol (identified by gas chromatography) are recovered from the off-gas by means of a cold trap. In order to remove any unreacted dimethyl maleate, the reaction mixture is stripped at 0.1 mm Hg pressure while stirring at 90"C. During this time another 2 parts of methanol are recovered, but no unreacted dimethyl maleate. Then 0.1 part p-methoxyphenol is added and the product is allowed to cool to room temperature. 230.2 parts of a resinous, clear, but slightly yellowish oil are obtained. It is calculated that the ratio (dimethyl maleate reaction)/(amine) is equal to 0.25.

EXAMPLE 2 This preparation is carried out as described in Example 1. 100 parts Versamid 115 are reacted with 40 parts dimethyl maleate for 3 hours at 90"C. On stripping under reduced pressure approximately 17 parts of dimethyl fumarate and approximately 4 parts of methanol are recovered (both components are identified by gas chromatography). After adding 0.05 part p-methoxyphenol, the product is allowed to cool at room temperature. 117 parts of a greenish-yellow resinous liquid are obtained. It is calculated that the ratio (dimethyl maleate reacted)/(amine) is equal to 0.35.

EXAMPLE 3 This preparation is carried out as described in Example 1.99.1 parts Versamid 115 and 40 parts dimethyl maleate are reacted at 900 C. for 5 hours. Approximately 6 parts methanol are formed during this time. After adding 0.05 part p-methoxyphenol, the reaction mixture is stripped under reduced pressure. Approximately 13 parts of dimethyl fumarate are recovered. 124 parts of a greenish resinous liquid are obtained. It is calculated that the ratio (dimethyl maleate reacted)/(amine) is equal to 0.52.

EXAMPLE 4 100 parts reactive polyamide resin (General Mills Chemicals, Versamid 100, amine number 90) and 30 parts diethyl maleate are reacted fro 5 hours as described in Example 1.

During this time 5 parts ethanol are formed. Then 0.05 part p-methoxyphenol is added. On stripping under reduced pressure an additional small amount of ethanol and 6 parts diethyl fumarate are recovered. 116 parts product are obtained as a residue. The product is a clear, yellowish resinous liquid. It is calculated that the ratio (diethyl maleate reacted)/(amine) is equal to 0.80.

EXAMPLE 5 100 parts reactive polyamide resin (Emery Industries, Emerez 1515; amine number 345) and 50 parts dimethyl maleate are reacted for 4 hours at 900C. as described in Example 1.

During this time 8 parts methanol are recovered from the off-gas in a cold trap. On stripping under reduced pressure 5 parts unreacted dimethyl fumarate are recovered. Then 0.05 part p-methoxyphenol is added and the product is taken as a residue. 135 parts clear, yellow resinous liquid are obtained. It is calculated that the ratio (dimethyl maleate reacted)/(amine) is equal to 0.51.

EXAMPLE 6 100 parts reactive polyamide resin (General Mills Chemicals, Versamid 115, amine number 238) and 15 parts dimethyl maleate are reacted for 3 hours as described in Example 1. After stripping at 900C and 0.1 mm Hg pressure, 20 parts of 5norbornene-2,3-dicarboxylic anhydride are added. The reaction mixture is stirred at 900C under a nitrogen blanket for another 3 hours. Then 0.05 part p-methoxyphenol is added and the reaction mixture is allowed to cool to room temperature. An extremely viscous resinous liquid is obtained. It is calculated that 24.5 % of the amine groups are reacted with dimethyl maleate and 28.7% with 5-norbornene-2,3-dicarboxylic anhydride.

EXAMPLE 7 100 parts Versamid 115 are reacted with 45 parts dimethyl itaconate for 3 hours at 900C as described in Example 1. Then the reaction mixture is stripped at 0.1 mm Hg pressure.

Approximately 3 parts of methanol and 20 parts of a mixture of dimethyl mesaconate, dimethyl itaconate and dimethyl citraconate (identified by gas chromatography) are removed from the reaction mixture. 0.05 parts p-methoxyphenol is added and the product is taken as a residue. 120 parts clear, yellowish viscous oil are obtained.

EXAMPLE 8 - UV Curable Overprint Varnish 100 parts of the product of Example 1, 135 parts trimethylolpropane triacrylate, 30 parts hydroxyethyl acrylate, 2 parts paraffin wax Esso (Registered Trade Mark) wax 3150; m.p.

135 C), and 10 parts benzoin isobutyl ether are mixed well on a roller mill. A clear, almost colourless, medium oil is obtained.

Films of 0,4 mil thickness are applied with a wire wound coating rod onto paper, aluminium foil, vinyl coated aluminium foil, polyester coated Mylar (Registered Trade Mark), and steel.

The coated substrates are exposed for 1/10 second to the UV radiation given off by a medium pressure mercury vapour lamp (Hanovia (Registered Trade Mark) 200 W/inch) at a distance of 5" from the lamp. This distance coincides with the second focal point created by the ellipitical reflector.

Afte; this exposure, all the coatings are cured. They have a pencil hardness of at least 3H and rub resistance of at least 40 rubs using methyl ethyl ketone as the solvent.

EXAMPLE 9 - Electron Beam Curable Coating 20 parts of reaction product from Example 2, 20 parts trimethylolpropane triacrylate, and 20 parts neopentyl glycol diacrylate are mixed on a roller mill. A clear, slightly yellowish medium oil is obtained. Steel, wood, aluminum and plastic panels are coated with this formulation using a 1 mil film knife. The coated test panels are exposed to 6 Mrad electron beam radiation under exclusion of air. After the irradiation, the coatings are clear, colorless, hard and glasslike. They have a pencil hardness of 4H, show excellent adhesion and are unaffected by solvents.

The same type of coatings are cured in presence of air by exposing them to 20 Mrad electron beam radiation.

EXAMPLE 10 - Electron Beam Curable Coating 20 parts product from Example 7, 10 parts trimethylolpropane triacrylate, 10 parts 1,6-hexane-diol diacrylate, 10 parts acrylated epoxidized soybean oil (Union Carbide Co., Actomer X-70), and 1 part stearyl acrylate are milled on a roller mill until homogeneous. A clear, almost colorless, medium viscosity oil is obtained. Coatings of 4 mil thickness are applied with a film knife to asphalt tile, vinyl tile and wood. The coated substrates are exposed to 5 Mrad electron beam radiation under exclusion of air. After the exposure the coatings are tough, clear colourless, not affected by solvents, and are very abrasion resistant.

EXAMPLE 11 - UV Curable Ink 50 parts of reaction product from Example 2, 20 parts trimethylolpropane triacrylate, 10 parts Lithol rubine pigment, and 3 parts stearyl acrylate are mixed and ground on a three-roll mill until homogeneous. Then 5.5 parts benzoin isobutyl ether, dissolved in 10 parts trimethylolpropane triacrylate are added and the milling is continued for a short time. The ink is applied to paper stock as a film of 0.1-0.4 mil thickness by means of a rubber roller.

Complete cure of the ink is achieved by exposure to UV radiation for 0.5 seconds as described in Example 8.

The radiation curable coatings of this invention are extremely useful for making hard coatings without the application of extensive heat to the coated surface, and without the necessity of depositing an already cured resin from a solvent which must then be evaporated and recovered. The modified prepolymers of the present invention make it possible to prepare hard coatings by spreading a prepolymer material of convenient fluidity and then quickly curing the film to make it resistant to abrasion and to solvents.

EXAMPLE 12 10 parts dimethyl maleate - modified reactive polyamide resin (Versamid 115 -- General Mills Chemicals -- reacted with 40% by weight of dimethyl maleate at 900 C. for 3 hours, excess dimethyl maleate removed under reduced pressure) are mixed with 10 parts pentaerythritol tetraacrylate, 0.01 part phenothiazine, 0.4 part Esso wax 3150 (paraffin wax, m.p. approx. 132"C.), 0.8 part benzoin isobutyl ether and 20 parts methylene chloride. A clear, low viscosity oil is obtained.

A carrier paper made of smooth clay coated sheet weighing about 16 pounds per ream (500 sheets, 20 x 25 inches) is coated with the above solution using a #20 Mayer rod. The coated paper then is exposed for 1/10 second to the UV radiation given off by a medium pressure mercury vapor lamp (Hanovia, 200 W/inch) at a distance of 5 inches from the lamp. This distance coincides with the second focal point created by the elliptical reflector. By this exposure, the coating is cured to a hard, glossy film which has a pencil hardness of 3H.

The radiation cured release coating is then print coated with a clear lacquer composed of 6 parts cellulose acetate - butyrate (Eastman EAB 171-40), 55 parts ethylacetate, 28 parts toluene, and 11 parts 95% ethanol (Printing grade). Print coating is accomplished using a regular varnish etch cylinder having a depth of about 20 to 40 microns, 120 line screen and a 15-20 wall (equivalent to #8 Mayer rod). The applied lacquer coating is dried at 1400F. for 1 minute.

The design print is then printed over the clear lacquer layer using polyamide - nitrocellulose modified ink containing pigment or dye of the colour desired (ZYROTO WHITE, sold by Gotham Ink and Color Co., which contains a titanium dioxide pigment). The area of the design print is smaller than that of the printed lacquer layer and falls wholly within the margin of the lacquer layer.

Over the ink, as an overprint, there is then coated a clear adhesive layer comprising a solution of heat-activatable thermoplastic polyamide resin in lacquer form (VERSAMIDE (Registered Trade Mark) 940, sold by General Mills) followed by drying the adhesive to a dry non-tacky state. The area of the adhesive overprint is smaller than that of the lacquer and it falls wholly within the margins of the lacquer layer.

There is no pick-up during the three printing operations.

The heat transfer label, as formed above, is then put in contact with a polyethylene bottle, the surface of which has been treated to render it more print receptive in a conventional manner such as by flame contact. Heat and pressure are applied to the temporary backing to effect pressing of the adhesive layer against the polyethylene surfaces. As heat is applied, approximately 350"F., there is no softening of the release layer or the cellulose acetate layer, but the adhesive overprint is heat-activated to a highly tacky state and bonds to the polyethylene surface of the bottle. The temporary backing may then be stripped from the transfer label or may be left on the transfer and stripped at a subsequent time without danger of delaminating the transfer from the polyethylene surface. No portion of the release layer is left over the transfer after stripping of the temporary backing and no portion of the lacquer is removed with the release layer. After cooling and peeling of the temporary backing, the bottle so coated is flame treated and the adherence of the label thereto is determined.

Adherence was excellent.

EXAMPLE 13 Same as Example 12, except that 1.0 part of cyclododecane is used in place of the Esso Wax 3150.

EXAMPLE 14 Same as Example 12, except that 1.0 part stearic acid is used in place of Esso Wax 3150.

EXAMPLE 15 Same as Example 12, except that 1.4 parts - methanacryl oxypropyl trimethoxy silane (Union Carbide (Registered Trade Mark) SILANE A-174) are used in place of the Esso Wax 3150.

EXAMPLE 16 Same as Example 12, except that 0.6 part E-wax (Farbwerke Hoechst AB, Montan type wax) is used in place of the Esso Wax 3150.

EXAMPLE 17 Same as Example 12, except that 1.0 part FL-wax (Farbwerke Hoechst AB, Montan type wax) is used in place of the Esso wax 3150.

EXAMPLE 18 Same as Example 12, except that 1.4 parts F-wax (Farbwerke Hoechst AB, Montan type wax) are used in place of the Esso wax 3150.

EXAMPLE 19 Same as Example 12, except that 0.5 part octadecanol is used in place of the Esso wax 3150.

EXAMPLE 20 Same as Example 12, except that 1.0 part oleamide (Armour Industrial Chemical Co., Armid (Registered Trade Mark) 0) is used in place of the Esso wax 3150.

EXAMPLE 21 Same as Example 12, except that 1.4 parts stearyl acrylate are used in place of the Esso wax 3150.

EXAMPLE 22 Same as Example 12, except that 0.6 part isodecyl acrylate is used in place of the Esso wax 3150.

EXAMPLE 23 Same as Example 12, except that 1.0 part Fluorolube LG-160 (Hooker Chemical Co.) is used in place of the Esso wax 3150.

EXAMPLE 24 Same as Example 12, except that 1.4 part halocarbon wax (Halocarbon Products Corp.) is used in place of the Esso wax 3150.

EXAMPLE 25 Same as Example 12, except that 1.0 part silicone S-30 (Union Carbide Co.) is used in place of the Esso wax 3150.

EXAMPLE 26 Same as Example 12, except that 1.0 part silicone L-31 (Union Carbide Co.) is used in place of the Esso wax 3150.

EXAMPLE 27 Same as Example 12, except that 0.5 part Syl-Off 291 (Dow Corning Co.) is used in place of the Esso wax 3150.

EXAMPLE 28 Same as Example 12, except that 1.0 part Syl-Off 291 containing 6% Catalyst 23A (Dow Corning Co.) is used in place of the Esso wax 3150.

EXAMPLE 29 Same as Example 12, except that 1.0 part V-wax (Farbwerke Hoechst AG, poly (octadecyl vinyl ether) is used in place of the Esso wax 3150.

EXAMPLE 30 Same as Example 12, except that 1.4 parts W-wax (Farbwerke Hoechst AG, hydrogenated animal fat) is used in place of the Esso wax 3150.

In the reactive polyamide prepolymers herein described, the polyamide preferably has primary amine end groups and at least one secondary amine group in the polymer chain. They may be prepared, for example, from essentially saturated dibasic acids such as dimerized fatty acids and triamines such as diethylene triamine, excess amine being employed to provide amine end groups. Each reactive polyamide resin molecule is then preferably reacted with at least three molecules of an unsaturated polybasic carboxylic acid, reaction being terminated when the acid molecule has formed one amide linkage and before significant cross-linking occurs. The unsaturnted poly-basic acid may be maleic, fumaric, itaconic or 5norbornene-2,3-dicarboxylic acids, or their lower alkyl ester or anhydrides where "lower" indicates a typical chain length of up to about 4. The unsaturated groups of these acids co-polymerize with radiation polymerizable vinyl monomers to provide radiation cured films, but do not react with the amine groups of the reactive polyamine when condensed therewith.

These prepolymers are especially useful as inks and can be diluted with vinyl monomers including acrylates, styrene vinyl ethers and vinyl acetate which can be radiation copolymerized therewith.

Claims (7)

WHAT WE CLAIM IS:

1. A radiation curable liquid prepolymer for preparing radiation curable composition, the prepolymer comprising the reaction product of a polyamide polyamine and an unsaturated polybasic carboxylic acid or an anhydride or lower alkyl ester of such an acid.

2. A liquid prepolymer as claimed in Claim 1 which includes an acrylic ester monomer copolymerizable by radiation with the said prepolymer to provide coating viscosity.

3. A liquid prepolymer as claimed in Claim 1 or Claim 2 which includes a photoinitiator for promoting rapid cure under exposure to radiation.

4. A liquid prepolymer as claimed in any of Claims 1 to 3 which includes a polymer to act as a strengthening and shrink control agent.

5. A method of making a liquid prepolymer as claimed in Claim 1 substantially as described herein with reference to any one of Examples 1 to 7.

6. A liquid prepolymer as claimed in Claim 1 substantially as described herein with reference to any one of Examples 1 to

7.